This invention relates to novel absorption pairs for absorption heating and cooling.
In view of diminishing fossil fuel supplies, and hence, increasing fuel costs, there is a need to minimize the amount of fuel society consumes to heat habitable space.
The heat pump concept, wherein available energy is taken from an ambient source such as outside air, and combined with fuel energy to heat space, is not new. Existing concepts include electrically driven-vapor compression heat pumps and absorption heat pumps. The latter require an absorption pair which comprises a solvent and a solute wherein the solvent remains a liquid, which may be a solution, throughout the operation of the apparatus, and the solute having a liquid and vapor phase in the cycles of the operation. The solute must be soluble in the solvent and must be readily separable as a vapor from the solvent by means of evaporation. In addition, the solute must be suitable for condensation from the vapor back to a liquid form. In general, all absorption heating apparatus require essentially the same parts and function in essentially the same way regardless of the particular solute and solvent used. Nevertheless, heat pumps as disclosed in U.S. Pat. Nos. 4,106,309, 4,127,009, 4,127,010 and 4,127,993 of B. A. Phillips are preferred. The major components of the apparatus are a generator, condenser, evaporator, absorber and absorption pair (also called absorber pair). The solute passes through all units and the solvent, sometimes also known as the absorbent, is confined to movement through the generator and absorber.
In operation, a mixture of absorbent and solute is heated in the generator to boil off most of all of the solute which rises as a vapor through a connecting conduit to the condenser. The mixture may be heated in the generator by any suitable means such as a gas flame, geothermal heat, solar heat or warm water.
The generator and condenser operate at relatively high pressure, so that condensing temperature of the solute is sufficiently high to permit rejecting the latent heat emitted by the condensing solute to outside air or cooling water passing through or around the condenser.
The liquid solute leaving the condenser passes through a conduit to a throttling valve (or its equivalent), through the throttling valve and through another conduit to the evaporator. The throttling valve throttles the liquid solute to a lower pressure so it will boil at a relatively low temperature in the evaporator and thus absorb heat from air or water passing through or around the evaporator.
The vaporized solute passes from the evaporator through a conduit to the absorber where heat of mixing is emitted (preferably to cooling water passing therethrough) as it is dissolved in cool absorbent which has been carried to the absorber by means of a conduit connecting the absorber with a generator outlet. The mixture of absorbent and solute resulting in the absorber then passes through a conduit to the generator where it is reheated to continue the process.
Any suitable material of construction for the apparatus may be used which can withstand the encountered temperature, pressure and corrosive properties, if any, of the solvent and solute. For the present compositions, aluminum, copper and their alloys are preferred. It is desirable, however, that minor components of a heat pump system (such as pump parts) be made of steel or other metals. Thus, it is desirable for the solute/solvent system to have good stability in contact with steel as well as with aluminum and copper.
Such a heat absorption apparatus is particularly desirable since moving parts, if any, are minimal when compared with the moving parts found in electrically driven-vapor compression heat pumps.
Unfortunately, the known solute/solvent systems for heat pumps have serious disadvantages. The most common solute/solvent pair (absorber or absorption pair) is ammonia/water. The ammonia/water pair has a disadvantage since the heating efficiency of apparatus utilizing the ammonia/water absorber pair is not as high as desired; i.e. the coefficient of performance (COP) practically attainable is generally less than about 1.30 at low generator temperature, i.e. below 180.degree. F., and at high generator temperatures, i.e. 220.degree. F., is generally below about 1.40. COP is a measure of the efficiency of the absorption cycle and is the ratio of the heat output to the energy input. The ammonia/water combination has additional disadvantages. Water is highly volatile, thus preventing complete separation of the ammonia from the water in the generator at high generator temperatures. The condensing pressure required to condense the ammonia is undesirably high, thus requiring equipment capable of withstanding such pressure.
The only other presently commercial absorber pair is water/lithium bromide wherein water is used as the solute and lithium bromide is used as the absorbent. The water/lithium bromide absorber pair (and the related water/lithium chloride absorber pair) has undesirable characteristics. For example, water as a solute is limited to an evaporation temperature of above about 32.degree. F., which is its freezing point. Lithium bromide is not sufficiently soluble in water to permit the absorber to be air cooled. The extremely low pressures in the system require large vapor conduits. Unless the system is precisely controlled, lithium bromide can crystallize and cause fouling of the system and the generator temperature cannot efficiently operate below 180.degree. F. nor above 215.degree. F. Additionally, aqueous lithium bromide solutions are corrosive, thus requiring special inhibitors and alloys for suitable apparatus.
Other absorber pairs which have been suggested have not been commercially accepted due to one or more disadvantages. Such disadvantages include a lack of sufficient affinity of the absorbent for the solute vapor, thus preventing sufficient absorption of the solute vapor to draw in and compress the solute. The absorber pairs have frequently not been mutually soluble over the whole range of operating conditions, thus permitting crystallization and the formation of solid articles which make it difficult or impossible for proper fluid circulation. The absorbent has frequently been too volatile, thus preventing the refrigerant vapor leaving the generator to be adequately purified. When absorbent evaporates from the generator, the efficiency of the system is frequently substantially reduced since energy input is wasted in evaporation. Additionally, the absorbent pairs previously suggested are frequently unstable, cause corrosion of the apparatus, are toxic or are highly flammable. Absorption pairs suggested in the prior art frequently have unacceptably high or unacceptably low working pressures. The working pressures should be as near to atmospheric pressure as possible to minimize equipment weight and minimize leaking into or out of the system. In addition, pressure difference between the high side and low side is frequently too high to facilitate circulation of the solution. The solutes suggested in the prior art frequently have a latent heat of evaporation which is unacceptably low, thus requiring large quantities of fluids to be circulated and the coefficient of performance of other absorber pairs suggested in the prior art is usually too low for serious consideration in commercial apparatus.
Some absorber pairs including a halogenated hydrocarbon solute (refrigerant) and an organic absorbent have been explored over the years for absorption refrigeration. Although certain specific absorber pairs wherein the absorbent included a furan-type ring had been proposed in U.S. Pat. No. 2,040,902 as a part of a program exploring numerous potential absorber pairs, no further discussion of furan-type absorbents has appeared in the art. Instead, subsequent exploratory work with organic absorbents has concentrated on acyclic glycol ethers and particularly on DMETEG (dimethoxytetraethylene glycol) and the ethyl ether of diethylene glycol acetate. More recently, and until the present invention, exploratory work on organic absorbents for halogenated hydrocarbon refrigerants has apparently lain dormant.